WATER
Handling water supply cross connection and backflow prevention with ease By Gilbert Welsford Jr.
U
tilities and government agencies dealing with potable water supply must protect piping systems from contamination. The presence of pathogens, suspended debris, and foul water poses a health risk to consumers. Backflow at cross connection is one of the common sources of contamination in potable water piping systems. As such, authorities and utilities need to carefully design and size piping systems and establish preventive measures against contamination. They also require robust maintenance programs to monitor the performance of different check valves, piping components and pipe connections. A typical public water supply system contains several backflow prevention devices at consumer terminals. As part of maintenance, regulatory authorities conduct regular tests to verify the reliability of the devices. They recommend repairs and replacement of damaged contamination control devices and evaluate the hazard levels of existing backflow prevention devices.
UNDERSTANDING CROSS CONNECTIONS AND BACKFLOW A cross connection can be a permanent or temporary connection between a potable public water supply line and a non-potable water source. It also refers to the potential connection between public water supply systems and pipes conveying wastewater, chemical products, sewage and stormwater drainage. Cross connections are available in heating, ventilation and cooling systems, fire suppression utilities, agricultural irrigation systems, chemical processing facilities and diverse factory floor equipment. A cross connection can cause undesirable backflows, which permit the flow of hazardous or pathogenic fluids into potable water systems. Backflow happens when there is a 14 | June 2022
A double-check valve backflow preventer. Credit: Simone, stock.adobe.com
reversed fluid flow. In the process, polluted substances get into the potable water system. Backflow is predominant at cross connections and occurs by back-siphonage, or backpressure backflow. BACK-SIPHONAGE A pressure difference must exist in the pipe systems for water to flow from the central distribution system to consumer premises. It could be from a water treatment plant to commercial or residential properties. The system maintains a positive pressure difference for the fluids to flow. There are a few instances when the public water distribution system experiences negative pressure. The existence of negative pressure creates a full or partial vacuum. When this happens, the probability of backflow increases. A negative pressure difference can occur if the supply is interrupted abruptly, experiences high demand, or if there is a break in the main water supply line. High-demand flow applications that can cause a break in the water supply line include the activation of a fire hydrant or
similar events. Back-siphonage allows water from the building’s plumbing system to freely flow back into the public water system to fill the existing vacuum. BACKPRESSURE BACKFLOW Non-potable water can flow into public water systems if there is a pressure imbalance in the system. Bad piping or poor pump selection and connection cause the downstream pressure to exceed upstream or the water supply pressure. When downstream pressure exceeds upstream pressure, fluids change their flow directions unless a suitable backflow prevention device is installed. In industrial processes, there is a probability that equipment experiences abnormal temperature changes. An increase in boiler temperature affects the pressure conditions of fluids flowing through them. As a consequence, water pressure in the boiler drops below the supply line pressure level. Such conditions make it easy for non-potable fluids to flow into potable water supply systems. Protecting public water supply from continued overleaf…
Environmental Science & Engineering Magazine
